Creatinine: (C4H7ON3) The nitrogenous waste product of phosphocreatine metabolism. It is a normal constituent of urine and blood. About .02 g/kg of body-weight of creatinine is excreted by the kidneys each day. Creatinine production is proportional to muscle mass and fairly consistent over time. Those who engage in vigorous exercise and those with higher muscle mass may have more creatinine appear in the urine. Increased quantities of creatinine are found in the urine during advanced stages of renal (kidney) disease.